The present invention relates to a method for anti-skid brake control for controlling braking force by adjusting braking force of a wheel when the wheel is tend to be locked and by eliminating the locking tendency, and in particular to a method for anti-skid brake control in case a wheel speed sensor for detecting wheel speed is not normally operated.
In anti-skid brake control, brake control is performed, when it is detected that a wheel is in skid tendency during braking, to stabilize driving of a vehicle and to make braking distance as short as possible by lowering braking force of the wheel and by eliminating skid tendency, and by increasing braking force again thereafter.
In the past, an anti-skid brake control unit with the anti-skid brake control applied on a vehicle braking device based on pneumatic pressure has been proposed, for example, in Japanese Patent Laid-Open Publication 50-96773.
FIG. 5 schematically shows a common example of a vehicle braking device equipped with a conventional type anti-skid brake control unit.
As shown in FIG. 5, the vehicle braking device 1 comprises an air tank 2 for accommodating compressed air, a brake valve 3 for controlling supply and discharge of compressed air to and from the air tank 2, a brake pedal 4 for controlling the brake valve 3, a brake booster 5 for generating braking hydraulic pressure by pressure of compressed air sent through the brake valve 3, a brake cylinder 6 for braking wheels by introducing hydraulic pressure converted by the brake booster 5, a controller 7 for performing anti-skid brake control, a wheel speed sensor 8 for detecting wheel speed and sending signal to the controller 7, and a modulator 9 for supplying, discharging or maintaining compressed air to and from the brake booster 5 by operating according to control signal sent from the controller 7 and for adjusting pneumatic pressure introduced into the brake booster 5.
By treading the brake pedal 4 and by operating the brake valve 3 during braking operation, output pressure of the brake valve 3 is supplied to the modulator 9. As the result, an opening valve (not shown) for holding of the modulator 9 is opened, and output pressure of the brake valve 3 is supplied to the brake booster 5, which generates braking hydraulic pressure. The braking hydraulic pressure generated at the brake booster 5 is introduced into the brake cylinder 6, which in turn operates the brake.
When the controller 7 judges during braking that a wheel is in locking tendency based on wheel speed sinal from the wheel speed sensor 8, the controller 7 operates a holding solenoid valve (not shown) of the modulator 9, which corresponds to the wheel in locking tendency.
As the result, a holding opening valve (not shown) is closed, and the brake is set to holding state. However, if locking tendency is not eliminated yet, an exhaust solenoid valve (not shown) of the modulator 9 is operated. Then, an exhaust opening valve is opened, and compressed air supplied to the brake booster 5 is discharged, and it leads to brake decompression state.
When locking tendency of the wheel is eliminated, the holding solenoid valve and the exhaust solenoid valve are returned to non-operating state. The exhaust opening valve is closed, and the holding opening valve is opened. Thus, it leads to brake pressurization state.
In this way, when the controller 7 calculates from the wheel speed sensor 8 and judges that the wheel is in locking tendency based on wheel speed, the holding solenoid valve and the exhaust solenoid valve of the modulator 9 are operated according to control signal from the controller 7. To the pneumatic pressure supplied from the brake booster 5, decompression, holding or pressurization are repeatedly controlled, and anti-skid brake control is performed to eliminate the locking tendency of the wheel.
The wheel speed sensor 8 in the anti-skid brake control of the vehicle braking device 1 generally comprises a sensor rotor 8b having a plurality of projections 8a integrally rotating with the wheel and a sensor 8c, which senses these projections and generates output. In this case, the gap between the sensor rotor 8b and the sensor 8c is adjusted approximately to 0 in a new vehicle.
On sensor signal inlet of the controller 7, an input filter (not shown) such as low pass filter is mounted to remove noise. Thus, the wheel speed signal from the wheel speed sensor 8 is removed of noise and is received by the controller 7. In case the input filter is provided in this way, sensor output characteristics of the sensor have frequency of A.sub.0 Hz or more as shown in FIG. 6 when the sensor is normal because of input sensitivity of the input filter. That is, the controller 7 begins to sense sensor output signal.
However, when the gap between the sensor rotor 8b and the sensor 8c is widened due to wearing, output voltage of the sensor 8c decreases. As the result, the frequency to start generation of sensor output voltage decreases to A.sub.1 Hz along input sensitivity line of input filter as shown by one-dot chain line. When the gap between the sensor rotor 8b and the sensor 8c is further widened and the frequency to start generation of sensor output voltage goes above the lower limit frequency A.sub.L (the frequency, at which anti-skid brake control is stopped if the sensor frequency approximately equal to wheel speed goes down below this frequency) of anti-skid brake control. Then, the controller 7 judges erroneously that the wheel is locked.
Possible cause for this erroneous judgment is as follows: As shown in FIG. 7, when braking is performed and wheel speed decreases under the condition where the frequency to start generation of sensor output voltage is above the lower limit frequency A.sub.L of anti-skid brake control, output voltage of the sensor is turned to 0 if output frequency of the sensor is below the frequency to start generation of sensor output voltage. Because the frequency to start generation of sensor output voltage is within the range to execute anti-skid brake control, the controller 7 erroneously judges that the wheel is locked because sensor output voltage is turned to 0.
When the controller 7 erroneously judges, exhaust signal is outputted to the modulator 9 so that exhaust operation for decompression of brake pressure is performed in anti-skid brake control, and the modulator 9 erroneously carries out exhaust operation. As the result, stopping distance is extended when the vehicle is braked, and this leads to poor driving performance.